This invention relates generally to a method and apparatus for removing artifacts in a vestigial sideband modulation (VSB) signal and more particularly to the removal of symbol clock related artifacts that may occur in a VSB signal when produced by an offset QAM (Phase Amplitude Modulation) generator.
It is well known that a QAM signal generator can be offset to generate a VSB signal. In such an arrangement, the I (in-phase) and Q (quadrature) signal components alternate in time, whereas in a conventional QAM signal, the I and Q signal components occur at the same time. The signal generated by a perfectly operating Offset QAM system is identical to, and cannot be distinguished from, a VSB signal, before the addition of the QAM carrier or the VSB pilot. The VSB pilot is placed 310 KHz from one edge of the 6 MHz band, whereas the QAM carrier is positioned in the center of the 6 MHz band. The symbol clock runs at 10.76 MHz and when the signal is demodulated from the pilot (in a VSB receiver) the center of the band is at 2.69 MHz (3.0 MHz minus 310 KHz). Therefore, any DC offset in the QAM generated VSB signal appears as a continuous wave (CW) of 2.69 MHz frequency, which represents one form of artifact. This artifact is at one-fourth of the clock frequency of 10.76 MHz and thus extends over (i.e., repeats) every four symbols. Another form of artifact is due to an imbalance in the I/Q components ratio of the mixers. This artifact repeats every two symbols and manifests itself in an amplitude tilt across the channel. These artifacts must be removed at either the transmitter or the receiver in order to provide a usable VSB signal. At the present time, the CW can be manually nulled out at the transmitter with a suitable control adjustment. However, the adjustment is very sensitive to drift and not suitable for practical applications. Artifacts due to an improper I/Q ratio are not presently dealt with.
The present invention provides a relatively simple, cost-effective method and apparatus for nulling out the artifacts that may appear in a VSB signal that is generated by an offset QAM generator. In one form of the invention, the artifacts are removed at the transmitter site and in another form of the invention the artifacts are eliminated at the individual VSB receiver site.
A principal object of the invention is to provide a novel VSB system and method.
Another object of the invention is to provide a VSB system that uses a VSB signal produced by an offset QAM generator.
A further object of the invention is to provide a method and apparatus for removing artifacts from an offset QAM generated VSB signal.